In general, as shown in FIG. 1, an air-conditioner system for a vehicle includes: a compressor 1 for compressing and discharging refrigerant; a condenser 2 for condensing the refrigerant of high pressure discharged from the compressor 1; an expansion valve 3 for throttling the refrigerant condensed and liquefied in the condenser 2; an evaporator 4 for exchanging heat between the liquefied refrigerant of low pressure throttled by the expansion valve 3 and evaporating the refrigerant to cool the air discharged to the inside of the vehicle due to heat absorption by evaporative latent heat; and a refrigerant pipe 5 to which the compressor 1, the condenser 2, the expansion valve 3 and the evaporator 4 are connected.
Moreover, a receiver drier (not shown) is mounted between the condenser 2 and the expansion valve 3 to separate the liquid-phase refrigerant from gas-phase refrigerant, so that just the liquid-phase refrigerant is supplied to the expansion valve 3.
The compressor 1 is a variable capacity swash plate type compressor, and it will be described in brief referring to FIG. 2. The variable capacity swash plate type compressor includes: a rotary shaft 10 which is mounted inside the compressor 1 and rotates by operation of an engine; a swash plate 13 which is mounted on the rotary shaft 10 in such a way that an inclination angle is varied, rotates together with the rotary shaft 10 and is connected with a plurality of pistons 14; and an Electronic control valve (ECV) 15 for controlling the inclination angle of the swash plate 13 in order to control an amount of refrigerant discharged.
The ECV 15 is operated by electric control and controls inside pressure of a crank chamber 11 by controlling a flow rate of the refrigerant while guiding the refrigerant of high pressure discharged from a discharge chamber 12. The inclination angle of the swash plate 13 decreases when the inside pressure of the crank chamber 11 increases, but increases when the inside pressure of the crank chamber 11 decreases.
Therefore, the inclination angle of the swash plate 13 is varied by duty control of the ECV 15, and an amount of refrigerant discharged from the compressor 1 is determined by the inclination angle of the swash plate 13. Finally, a load of the air conditioner can be reduced through the duty control of the ECV 15.
In the meantime, the expansion valve 3 may be a mechanical valve or an electronic expansion valve. Hereinafter, as an example, the electronic expansion valve will be described.
The air-conditioner system controls a superheat degree by controlling the flow rate of the refrigerant through control of the compressor 1 and the electronic expansion valve 3 depending on the load of the air conditioner.
In other words, the air-conditioner system sets a target superheat degree according to outdoor temperature or indoor temperature of the vehicle and controls the flow rate of the refrigerant by the control of an ECV duty of the compressor 1 or variation of a degree of opening of the electronic expansion valve 3 in order to converge the target superheat degree.
However, the air-conditioner system causes a change in a superheat degree due to a sudden increase of sensitivity when the electronic expansion valve 3 controls the flow rate of the refrigerant (control of the superheat degree) in a variable area of the compressor 1, for example, an decrease area of the amount of refrigerant discharged from the compressor 1 in a low load condition.
In this instance, when the superheat degree is changed, cooling performance is also changed, and it changes the variable quantity (refrigerant discharge amount) of the compressor 1. Under such a condition, if the degree of opening of the electronic expansion valve 3 is also changed, it is difficult to control the flow rate of the refrigerant and it increases change in the flow rate of the refrigerant. Because of such problems, hunting that the refrigerant beats and generates pulsation occurs, and it causes instability of the system.
FIG. 3 is a graph showing the superheat degree, temperature of air discharged to the inside of the vehicle and the ECV duty of the compressor when the superheat degree is controlled through an electronic expansion valve in the variable area of the compressor. It will be described in brief.
First, in a superheat degree increase section (A), the temperature (° C.) of air discharged to the inside of the vehicle and the ECV duty (%), namely refrigerant discharge amount, of the compressor 1 are increased, and in this instance, the electronic expansion valve 3 greatly increases the degree of opening (refrigerant flow rate) in order to drop the superheat degree.
When the degree of opening of the electronic expansion valve 3 is greatly increased, the superheat degree lowers. In a section (B) where the superheat degree lowers, the temperature (° C.) of air discharged to the inside of the vehicle and the ECV duty (%), namely refrigerant discharge amount, of the compressor 1 are decreased. In this instance, when temperature (° C.) of air discharged from the evaporator 4 drops below zero, the air-conditioner system cuts the compressor 1 off in order to prevent icing of the evaporator 4. When the compressor 1 is turned off, the temperature of the air discharged to the inside of the vehicle increases while the temperature of the air discharged from the evaporator 4 increases, hence cooling performance is deteriorated.
Continuously, when the compressor 1 is turned on due to an increase of the temperature of the air discharged from the evaporator 4, the temperature of the air discharged from the evaporator 4 drops below zero, and then, the compressor 1 is turned off again. This is why the system becomes unstable.
In such an air-conditioner system, cooling performance is deteriorated in a superheat degree over-increase section where the superheat degree increases above the optimum superheat degree (optimum refrigerant flow rate). The refrigerant flow rate in the low load condition (the variable area of the compressor) of the air-conditioner system is smaller than the refrigerant flow rate in a high load condition, a control range of the superheat degree is narrow. Therefore, a control range of the optimum superheat degree area becomes narrow, hence instability possibility of the system increases when a common target superheat degree is controlled.
In other words, the conventional air-conditioner system has a disadvantage in that the system becomes unstable when the target superheat degree is controlled through the electronic expansion valve 3 in the same way as the high load condition, namely at the time of the maximum discharge of the compressor, even though the refrigerant discharge amount is small in the low load condition of the air-conditioner system, namely in the variable area of the compressor 1 where the ECV duty lowers.